1. Field of the Invention
This invention relates to a process for gas phase electrochemical reduction of CO.sub.2 and/or CO to CH.sub.4 and C.sub.2 H.sub.4 at ambient temperatures. The process is carried out at a solid polymer electrolyte wherein a metal electocatalyst capable of providing adsorption sites for CO.sub.2 and/or CO and chemisorbed hydrogen species or faradaically generated hydrogen species in proximity to the adsorbed CO.sub.2 and/or CO is deposited on one side of the solid polymer electrolyte to function as a cathode. CO.sub.2 and/or CO is passed in contact with the electrocathode while hydrogen ion is passed through the solid polymer electrolyte from an anode portion of the cell capable of providing hydrogen. Formation of hydrocarbons occurs during electrochemical reduction of CO.sub.2 and CO at the metal/solid polymer electrolyte interface.
2. Description of the Prior Art
Considerable effort has been expended towards promoting the electrochemical reduction of CO.sub.2 to useful hydrocarbons at both high Faradaic efficiencies and high current densities.
Indirect reduction of CO.sub.2 on a mercury electrode in an aqueous electrolyte, pH 7, containing TiCl.sub.3, Na.sub.2 MoO.sub.4 and pyrocatechol where the total Faradaic efficiency for cathodic hydrocarbon generation was about 0.2 percent at 7 mA/cm.sup.2, with methane being the major hydrocarbon component is taught by Petrova, G. N. and O. N. Efimora, Elektrokhimiya, 19(7), 978 (1983). CO.sub.2 has been shown to be reducible to CH.sub.4, CO, and methanol at ruthenium cathodes in CO.sub.2 saturated aqueous Na.sub.2 SO.sub.4 electrolyte with Faradaic efficiencies for CH.sub.4 production up to 42 percent at current densities up 0.11 mA/cm.sup.2 by Frese, Jr., K. W. and S. Leach, J. Electrochem. Soc., 132, 259 (1985).
Copper, 99.99 percent pure, was used as a cathode with 0.5M KHCO.sub.3 electrolyte for the electrochemical reduction of CO.sub.2 at ambient temperature and current density of 5.0mA/cm.sup.2 for 30 to 60 minutes with Faradaic efficiencies for CH.sub.4 of 37 to 40 percent, Hori, Y, K. Kikuchi and S. Suzuki, Chem. Lett., 1695 (1985). In later work high purity copper cathodes, 99.999 percent, were used for the electrochemical reduction of CO.sub.2 in 0.5M KHCO.sub.3 electrolyte in a cell operated at a current of 5mA/cm.sup.2 for 30 minutes at temperatures of 0.degree. C. and 40.degree. C. shows Faradaic efficiency for production of CH.sub.4 drops from 60 percent at 0.degree. to 5 percent at 40.degree. ; C.sub.2 H.sub.4 increases from 3 percent at 0.degree. to 18 percent at 40.degree. ; while hydrogen production increases from 20 percent at 0.degree. to 45 percent at 40.degree. . It is stated that 99.99 percent pure copper cut the Faradaic efficiencies to about one-third of those obtained with 99.999 percent pure copper. Hori, Y, K. Kikuchi, A. Murata and S. Suzuki, Chem. Lett., 897 (1986). Later work of electrochemical reduction of CO.sub.2 at a 99.999 percent pure copper cathode in aqueous electrolytes of KCl, KClO.sub.4 and K.sub.2 SO.sub.4 at 19.degree. C. and current density of5mA/cm.sup.-2 showed Faradaic yields of C.sub.2 H.sub.4 of as high as in the order of 48 percent, CH.sub.4 12 percent and EtOH 21 percent. Hori, Y, A. Murata, R. Takahashi and S. Suzuki, J. Chem. Soc., Chem. Commun, 17, 1988.
Electroreduction of CO at a 99.999 percent pure copper cathode in an aqueous catholyte of KHCO.sub.3 at ambient temperature for 30 minutes showed hydrogen the predominant product and at 1.OmA/cm.sup.2 C.sub.2 H.sub.4 Faradaic production was 22 percent, CH.sub.4 1 percent; 2.5mA/cm.sup.2 C.sub.2 H.sub.4 Faradaic production was 21 percent, CH.sub.4 16 percent and at 5.0mA/cm.sup.2 C.sub.2 H.sub.4 Faradaic production was 16 percent, CH.sub.4 6 percent Hori, Y, A. Murata, R. Takahashi and S. Suzuki, J. Am. Chem. Soc., 109. 5022 (1987). Similar work by the same authors showed electroreduction of CO at a 99.999 percent pure copper cathode in an aqueous 0.1 M KHCO.sub.3 pH 9.6 catholyte at 19.degree. C. at 2.5mA/cm.sup.2 resulted in Faradaic production C.sub.2 H.sub.4 of 21.2 percent; CH.sub.4 of 16.3 percent; EtOH of 10.9 percent; and 45.5 percent H.sub.2. Hori, Y, A. Murata, R. Takahashi and S. Suzuki, Chem. Lett., 1665 (1987).
In the reduction of CO.sub.2 to CH.sub.4 using 99.9 percent pure cold rolled B 370 copper cathodes with a CO.sub.2 saturated 0.5M KHCO.sub.3 electrolyte, Faradaic efficiencies of 33 percent were achieved for CH.sub.4 at current densities up to 38 mA/cm.sup.2 with no C.sub.2 H.sub.4 being detected. Cook, R. L. R. C. McDuff and A. F. Sammells, J. Electrochem. Soc., 134, 1873 (1987).
Electrochemical reduction of CO.sub.2 to CH.sub.4 and C.sub.2 H.sub.4 was shown to occur at copper/NAFION perfluorinated sulfonic acid copolymer electrodes (solid polymer electrolyte structures) at Faradaic efficiencies of about 9 percent for each CH.sub.4 and C.sub.2 H.sub.4 at E=-200 V vs. SCE using 2 mM H.sub.2 SO.sub.4 counter solution at a temperature of 22.degree. C. Dewulf, D. W., A. J. Bard, Cat. Lett. 1, 73, (1988). The Dewulf et al article was received Jan. 4, 1988 and published in February 1988 while a portion of the present invention was published in June 1988 from a manuscript submitted Oct. 7, 1987 and a revised manuscript received Dec. 17, 1987: Cook, R. L., R. C. MacDuff and A. F. Sammells, J. Electrochem. Soc., 135, 1470 (1988).